Re: Problems with FindRoot and recursive functions

• To: mathgroup at smc.vnet.net
• Subject: [mg89672] Re: Problems with FindRoot and recursive functions
• From: dh <dh at metrohm.ch>
• Date: Tue, 17 Jun 2008 00:38:44 -0400 (EDT)
• References: <g35g0m\$9bv\$1@smc.vnet.net>

```
Hi Daniel,

this is a very common mistake. For speed, Mathematica tries first to

simpify your expression symbolically. Therefore, you should take care

not to evaluate your function  with a symbolic argument. This can be

done e.g. by:

x1[t_?NumericalQ]:=...

hope this helps, Daniel

Daniel Kuang Chen Liu wrote:

> Hello,

>

> I have a recursive function of the form

>

>>> x1[t_] := If[t < 0, {t, 1}, 0.5 + x1[t - 1]]

>

> which returns a list of length 2, and the first element has a root at t=0.5

>>> In[3]:= x1[0.5]

>>> Out[3]= {0., 1.5}

>

> I want to use FindRoot to determine t0 such that x1[t0][[1]] == 0, so I use

> the following code

>

>>> FindRoot[x1[t][[1]] == 0, {t, 0.5}]

>

> to which Mathematica complains

>

> During evaluation of In[6]:= FindRoot::nlnum: The function value

> {False} is not a list of numbers with dimensions {1} at {t} = {0.5}.

>

> It would much appreciated if someone could tell me what is wrong with the

> code.

>

> Daniel Liu

>

>

--

Daniel Huber

Metrohm Ltd.

Oberdorfstr. 68

CH-9100 Herisau

Tel. +41 71 353 8585, Fax +41 71 353 8907

E-Mail:<mailto:dh at metrohm.com>

Internet:<http://www.metrohm.com>

```

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